Someone explained it to me with two circles of how close the different cultures let someone in: in the U.S. it's pretty easy to get through that first layer and make some acquaintances, but deep friendships may take longer. Germans are not big into casual acquaintances, but once you're in that first circle, it's easier to get through to the second one as well.

There's also a difference in push/pull: Germans will often expect you as the new guy to make the first steps, whereas that will be the opposite in other cultures that are more inviting.

Joining some Verein or taking a class at the Volkshochschule can help crossing that first circle.